High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations can be prepared by conventional melt polymerizations. Solid state polymerization is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers is eliminated during the solid state polymerization phase. Thermal degradation during the solid state portion of the polymerization is also essentially avoided. By utilizing solid state polymerization techniques significant cost savings can often be realized.
In melt polymerizations, molecular weight increases by esterification as well as transesterification. During the initial stages of melt polymerizations which utilize diacids, such as terephthalic acid, esterification reactions are predominate. Transesterification reactions are predominant during the initial stages of melt polymerizations which utilize diesters of diacids, such as dimethyl terephthalate. As the molecular weight of the polyester increases by esterification, most of the carboxyl end groups are consumed by esterification and water is eliminated. In transesterification reactions, alkyl end groups are consumed with alcohols being produced as reaction by-products. For example, the transesterification of dimethyl terephthalate results in the production of methanol as a reaction by-product. The final stages of such polymerizations are predominated by polycondensation reactions with the generation of glycol as a reaction by-product. The attainment of high molecular weights in melt polymerizations is hindered by a reduced rate of diffusion of the glycol due to the increased viscosity of the molten polymer.
In the solid state polymerization of standard pellets or chips having a relatively low surface area per unit weight, the polymerization reaction proceeds primarily by esterification with the diffusion of water. Transesterification leads to slower solid state polymerization rates because glycol by-products diffuse from the pellets or chips more slowly than water. However, polyester prepolymer in the form of powders can be solid state polymerized at a fast rate because of the high surface area and reduced path for glycol removal.
The low molecular weight polyester prepolymers utilized in solid state polymerizations are generally in the form of pellets, chips, or finely divided powder. Such pellets can vary greatly in size; however, as a general rule, the smaller the size of the pellets of polyester prepolymer the faster the solid state polymerization will proceed. Such polyester prepolymers are generally converted from the amorphous to the crystalline state prior to solid state polymerization in order to raise their sticking temperature. This is done in order to keep the pellets or chips of polyester prepolymer from sticking together as a solid mass in the solid state polymerization reactor.
In the solid state polymerization of a polyester prepolymer the polymerization is carried out at an elevated temperature which is below the melting point of the polyester resin. Such polymerizations are normally conducted in the presence of a stream of inert gas or under a vacuum. Solid state polymerizations are normally conducted on a commercial basis in the presence of a stream of inert gas since it serves to remove volatile reaction products and helps to heat the polyester.
Heretofore, the form of the polyester prepolymer has essentially dictated the type of solid state polymerization process which could be employed in order to convert the low molecular weight polyester prepolymer into high molecular weight polyester resin. For example, it has generally been accepted practice to solid state polymerize polyester prepolymer in the form of pellets or chips in vacuum or in an inert gas stream using a batch or a continuous process. Fluidized bed processes are generally the best means to solid state powders of polyester prepolymer. The reason for this is that experience has shown that finely ground powders tend to agglomerate in vacuum processes, resulting in slower polymerization rates and a need to regrind the high molecular weight polyester resin produced. Experience has also shown that, in static bed and moving bed processes finely ground powders will channel or fissure, resulting in uneven polymerization and prolonged polymerization rates. On the other hand, the use of pellets or chips in fluidized bed processes is not economically feasible in view of the velocity and volume of inert gas needed to suspend the pellets or chips and the size of the equipment required to do so.
Polyester prepolymers which are in the form of finely divided powders solid state polymerize at faster rates than do polyester prepolymers which are in the form of pellets or chips. However, polyester prepolymers which are in powder form are difficult to handle and generally must be polymerized in fluidized bed processes. Additionally, the high molecular weight polyester resins which are made utilizing prepolymers which are in powder form are also in the form of powders which are more difficult to process into articles of manufacture. For these reasons polyester prepolymers in powder form have not been widely utilized in commercial solid state polymerization techniques.
U.S. patent application Ser. No. 07/030,319 indicates that polyester prepolymers in the form of porous pills can be solid state polymerized at very fast polymerization rates with the high molecular weight polymer produced having a very narrow molecular weight distribution. In fact, polyester prepolymers in the form of porous pills can be solid state polymerized at a rate which is essentially equivalent to the rate at which powdered prepolymers can be solid state polymerized. Such polyester prepolymers in the form of porous pills can be solid state polymerized in virtually any type of reaction zone, such as a static bed or a fluidized bed. The resultant high molecular weight polyester resin produced can be processed in conventional equipment which is designed to accept standard pellets or chips of the high molecular weight polyester resin. By solid state polymerizing porous pills of polyester prepolymer essentially all of the advantages associated with using pellets or chips are realized without being subjected to slow polymerization rates.